1. Field of the Invention
The present invention relates to a semiconductor laser, a semiconductor light emitting device, andmethods of manufacturing the same and, more particularly, a semiconductor laser which has a feature in configuration to reduce a threshold current density in a semiconductor laser using a nitride compound semiconductor, and a method of manufacturing the same.
2. Description of the Prior Art
In the prior art, a short wavelength semiconductor laser has been employed as a light source of an optical disk, a DVD (digital versatil disk) drive, etc. Since a recording density of the optical disk is in inverse proportion to square of a wavelength of a laser beam, a semiconductor laser which is able to emit a laser beam of shorter wavelength is requested. A currently available semiconductor laser which is able to emit a shortest wavelength is a red-light emitting semiconductor laser which has a wavelength of about 630 to 650 nm and is incorporated into the DVD drive put on sale in last year.
However, in order to enhance the recording density higher in a photo memory device, a shorter wavelength of an output light is needed. By way of example, a blue-light semiconductor laser which has a wavelength or around 400 nm is indispensable for recording moving pictures on the optical disk for two hours. Hence, in recent years, the short wavelength emitting semiconductor laser which has a wavelength in a blue light emitting range has been developed actively as a next generation optical disk light source.
As material for the blue-light emitting semiconductor laser mentioned above, ZnSe (Zinc selen) system in group II-VI compound semiconductor and GaN system in group III-V compound semiconductor have been studied. Since the ZnSe system can substantially lattice-match with Gays which has attained great actual results as a high quality substrate, it has been considered for a long time that the ZnSe system is more advantageous than the GaN system. Therefore, most of the research scholars in the world have been engaged with study of the ZnSe system and thus the ZnSe system has gone ahead in the field of semiconductor laser study.
As for the ZnSe system, room-temperature continuous-wave oscillation (CW oscillation) due to the injection excitation has already been reported. However, since essentially the ZnSe system is material which is ready to deteriorate, its reliability becomes an issue and therefore the ZnSe system has not yet come up to practical implementation.
In contrast, in the case of the GaN system, after the announcement of the GaN (gallium nitride) high luminance LED being manufactured by Nichia Chemical Co., Ltd. at the end of 1993 which acts as the boundary, the GaN being excellent in environment resistance such as reliability which is the bottleneck of the ZnSe system has been looked at again and the number of the research scholars in the world has been risen largely.
Then, study of the GaN system has advanced rapidly since the success of pulse laser oscillation has been reported similarly by Nichia Chemical Co., Ltd. early in December, 1995. After oscillation duration of 35 hours has been reported in the room-temperature continuous-wave oscillation (CW oscillation), currently the oscillation duration of 10,000 hours has been presumed in an accelerated test.
Next, the short wavelength light emitting semiconductor device in the prior art will be explained with reference to FIGS. 1, 2 and FIG. 3 hereinbelow. FIG. 1 is a schematic sectional view, taken along its optical axis, showing the short wavelength emitting semiconductor laser in the prior art, and FIG. 2 is a schematic sectional view showing a short wavelength light emitting diode in the prior art. FIG. 3 is a schematic sectional view showing the short wavelength emitting semiconductor laser having a different buffer layer structure in the prior art.
The semicnductor laser is formed as follows.
First, as shown in FIG. 1, a GaN buffer layer 812, an n-type GaN buffer layer 813, an n-type In0.1Ga0.9N (indium gallium nitride) layer 814, an n-type Al0.15Ga0.85N (aluminum gallium nitride) cladding layer 815, an n-type GaN optical guiding layer 816, an InGaN MQW (multi-quantum well) active layer 817, a p-type Al0.2Ga0.8N layer 818, a p-type GaN optical guiding layer 819, a p-type Al0.15Ga0.85N cladding layer 820, and a p-type GaN contact layer 821 are epitaxially grown in sequence on a sapphire substrate 811 having a (0001) plane as a principal plane, by an MOVPE (metal organic vapor-phase epitaxy) method.
Then, a part of the n-type GaN buffer layer 813 is exposed by means of dry etching, then an n-side electrode 822 made of Ti/Au (titanium/gold) is formed on an exposed surface and also a p-side electrode 823 made of Ni/Au (nickel/gold) is formed on the p-type GaN contact layer 821, Then, a pair of parallel and surfaces are formed by applying dry etching. A pulse laser oscillation can be achieved successfully by adopting the end surfaces as resonator faces. If necessary, refer S. Nakamura et al.; Japanese Journal of Applied Physics, vol.35, p.L74, 1996).
In FIG. 2, in the case of the light emitting diode, the GaN buffer layer 812, an n-type GaN layer 824, an n-type or p-type In0.15Ga0.85N active layer 825, and a p-type GaN layer 826 are epitaxially grown on the sapphire substrate 811 by the MOVPE method.
In this case, in order to obtain a practical luminescence brightness as the light emitting diode which can be operated by virtue of low injection, a Si (silicon) concentration or Zn (zinc) concentration in the In0.15Ga0.85N active layer 825 must be set to 1xc3x971017 to 1xc3x971021 atoms/cm3 and also a thickness of the In0.15Ga0.85N active layer 825 must be set to 1 to 500 nm, more preferably 10 to 100 nm If necessary, refer Patent application Publication (KOKAI) Hei 6-260682 and Patent application Publication (KOKAI) Hei 6-260683.
FIG. 3 is a sectional view, taken along its optical axis, showing another short wavelength semiconductor laser in the prior art. First, a GaN buffer layer 832, an n-type GaN intermediate layer 833, an n-type Al0.09Ga0.91N cladding layer 834, an n-type GaN optical guiding layer 835, an MQW active layer 836, a p-type Al0.18Ga0.82N overflow preventing layer 837, a p-type GaN optical guiding layer 836, a p-type Al0.09Ga0.91N cladding layer 839, and a p-type GaN contact layer 840 are epitaxially grown in sequence on a sapphire substrate 831 having the (0001) plane as the principal plane, by the MOVPE method.
Then, like the case in FIG. 1, the p-type GaN contact layer 840 and the p-type Al0.09Ga0.91N cladding layer 839 are mesa-etched by virtue of dry etching, then a part of the n-type GaN intermediate layer 833 is exposed by means of dry etching, then an n-side electrode 841 made of Ti/Au is provided on an exposed surface of the n-type GaN intermediate layer 833 and also a p-side electrode 843 made of Ni/Au is provided on the p-type GandN contact layer 840 via a stripe-like opening or a SiO2 (silicon oxidation) film 842. Then, a pair of parallel end surfaces acting as resonator faces respectively are formed by applying dry etching.
In addition, it has been proposed that the overflow preventing layer, i.e., the carrier stopper layer is provided to the n-side layer side. If necessary, refer Patent application Publication (KOKAI) Hei 10-56236. In this case, an n-type Si doped Al0.15Ga0.85N layer as a hole stopper layer, whose n-type impurity concentration is 1xc3x971019 atoms/cm3, and a p-type Mg doped Al0.15Ga0.85N layer as an electron stopper layer, whose p-type impurity concentration is 5xc3x971019 atoms/cm3, are provided between the active layers and the optical guiding layers respectively. At that time, the growth temperature is 1100xc2x0 C. which is a usual temperature used to grow GaN or ALGWN.
However, in the case of the short wavelength semiconductor laser in the prior art, there has been such a problem that the threshold current density is very large such as about 3.6 kA/cm2. This is because material of the nitride compound semiconductor needs essentially a large carrier density to generate the optical gain.
More particularly, this is because group Ill-V compound semiconductor having a zincblende crystal structure such as AlGaAs system, AlInGaAs system, etc. is used in the semiconductor laser which has been practically used in the prior art, whereas nitride compound semiconductor has a hexagonal crystal wurtzite structure having a very wide forbidden bandwidth and also has physical properties totally different from those of zincblende crystal material.
It is an object of the present invention to reduce a threshold current density in a short wavelength semiconductor laser using a nitride compound semiconductor.
For easy understanding of the present invention, a layer structure of a semiconductor laser will be shown in FIG. 4. In the following explanation, the term xe2x80x9cn-sidexe2x80x9d means that the position is deviated toward the n-side electrode from a center of the active layer, while the term xe2x80x9cp-sidexe2x80x9d means that the position is deviated toward the p-side electrode from the center of the active layer.
(1) According to the present invention, in the semiconductor laser using the nitride compound semiconductor, a structure is employed wherein the active layer which is positioned between the cladding layers 2 and 6 is composed of a single gain layer having a thickness of more than 3 nm and optical guiding layers 3, 5 are provided between the active layer 4 and the cladding layers 2, 6 respectively.
In this manner, if the active layer is composed of the single gain layer, the injection current into the active layer 4 is utilized effectively and therefore the threshold current density Jth can be reduced. In this case, since an optical confinement factor xcex93 becomes small if the thickness of the active layer 4 is below 3 nm and thus the threshold current density Jth is increased, it is effective to set the thickness of the active layer 4 to more than 3 nm. The single gain layer may be or may not be formed of the quantum well structure, but the single quantum well layer should be utilized if the quantum well structure is employed in this case. The single quantum well structure is called SQW hereinafter and the multi-quantum well structure is called MQW hereinafter.
For example, in the semiconductor laser made of the nitride compound semiconductor, in case the cavity loss is small such as 100 cmxe2x88x921, the threshold current density Jth by which the modal gain Gm can exceed the cavity loss to thus commence the laser oscillation is different according to the number of the quantum well layers in the gain layer. If the number of the quantum well layers in the gain layer is set to n (n is a natural number), the magnitude of the threshold current density Jth (n layers) can be given in the following order.
Jth (one layer) less than Jth (two layers) less than Jth (three layers) less than Jth (four layers) less than Jth (five layers)
In other words, if the total thickness of the active-layer 4 is set a fixed amount, the threshold current density can be reduced as the layer number of the quantum well layers constituting the active layer 4 is made small. The threshold current density can be most reduced if the active layer is composed of the single layer. An example of the characteristic curve showing a relationship between the current density and the modal gain by using the number of the quantum well layer as a parameter is illustrated in FIG. 5.
In the present invention, an SCH (separate confinement Heterostructure) structure in which the optical guiding layers 3, 5 are formed between the active layer 4 and the cladding layers (barrier layers) 2, 6 is employed. Hence, an optical confinement factor xcex93 of the active layer 4 can be increased and therefore the threshold Fermi level EFth can be lowered. As a result, the threshold current density Jth of the short wavelength semiconductor laser can be lowered and further overflow of the electron in the active layer 4 can be reduced to thus improve the efficiency.
According to the present invention, in the semiconductor laser using the nitride compound semiconductor, the active layer 4 is composed of the single gain layer hating a thickness of more than 3 nm and such active layer 4 is made of the undoped layer.
In this manner, if the active layer 4 is formed of the undoped layer, the impurity scattering can be made small and thus the hole mobility can be enhanced. As a consequence, the holes can be injected more uniformly into the active layer 4.
In this case, it is preferable that the impurity concentration of the undoped layer should be below 1.0xc3x971017 atoms/cm3. In the case of the undoped layer, sometimes the impurity contained in the upper and lower cladding layers 2, 6 is doped automatically into the active layer 4. In such case, resultantly the impurity is sometimes contained in the undoped layer at the concentration of below 1.0xc3x971017 atoms/cm3.
It is preferable that the thickness of the above gain layer should be set to more than 6 nm though it gives both merit and demerit as described follows.
That is that when the thickness of the above gain layer should be set to more than 6 nm, it gives merit that an optical confinement of the gain layer is increased to be lowered the threshold of the Fermi level and as the result reduce the decrease of the overflow current, and demerit that the threshold Jth is increased.
Also, it is preferable that the thickness of the above gain layer should be set to less than 30 nm. The modal gain Gm is increased with the increase of the thickness of the gain layer. However, since the threshold current density Jth has the relation of Jth=Nthxc2x7dxc2x7e/xcfx84s, (Nth is the threshold carrier density, d is a thickness of the active layer, e is an elementary charge.) an upper limit of the thickness of the gain layer which can use effectively the current thereinto is about 30 nm. In this point, the thickness of the gain layer may be set to 3 to 30 nm.
Also, in the present invention, the thickness of the gain layer is preferably set to below 10 nm.
The thickness of the gain layer may be set to below 30 nm, since the thickness of the above gain layer gives demerit of the increasing of the threshold current density Jth and merit of the lowering the threshold of the Fermi as described above.
In the SCH structure of the single quanta well structure, barrier layers are provided between the gain layer and the optical guiding layers 3, 5 respectively.
If the barrier layers are provided on and below the gain layer, the gain layer can serve as the well layer irrespective of the band structure of the optical guiding layers 3, 5, whereby a single quantum well structure can be constructed. In order to enhance the carrier injection efficiency, the forbidden bandwidth of the barrier layer may be set narrower than those of the optical guiding layers 3, 5. In case carrier confinement is regarded as the most important item, the forbidden bandwidth of the barrier layer may be aet wider than those of the optical guiding layers 3, 5. The forbidden bandwidth is also called an energy band gap.
(2) According to the present invention, when the cavity loss (threshold gain) of semiconductor laser is larger, in the semiconductor laser using the nitride corvound semiconductor, the active layer is composed of a multi-quantum well structure having two layers of gain layer ad shown in FIG. 5.
As stated above, if the layer number of the quantum well layer is set at 2, the threshold current density Jth can be reduced smaller in the multi-quantum well structure.
In addition, when the cavity loss (threshold gain) of semiconductor laser is much larger, the layer number of the quantum well layer being set at 3 can be reduced further smaller in the multi-quantum well structure.
According to the present invention, in the semiconductor laser using the nitride compound semiconductor, the active layer is composed of the multi-quantu well structure having two or three layers of gain layer, each layer having a thickness of more than 6 nm.
For instance, as shown in FIG. 5, in case the cavity loss is increased like 200 cmxe2x88x921, the threshold current density Jth by which the modal gain Gm exceeds the cavity loss to start the laser oscillation becomes Jth (two layers) less than Jth (three layers)≈Jth (one layer) less than Jth (four layers) less than Jth (five layers) The threshold current density Jth can be reduced in the case that the multi-quantum well structure in which the gain layer is composed of two layers is employed.
In addition, the above gain layer is composed of the undoped layer. Like this, if the multi-quantum well structure active layer is employed, it is desired that the gain layer is formed of the undoped layer and thus the hole mobility should be enhanced by reducing the impurity scattering. As a result, the holes can be injected more uniformly.
Also, it is preferable that the above gain layer 4 should be formed of the nitride compound semiconductor such as GaN, InGaN, AlGaN, AlInGaN, or the like, i.e., AlxGayIn1xe2x88x92xxe2x88x92yN where 0xe2x89xa6x less than 1, 0 less than yxe2x89xa61).
(3) According to the present invention, in the semiconductor laser using the nitride compound semiconductor, a multi quantum well structure is employed as the active layer and also the maximum position of the emitted light intensity distribution is shiftedtowardthep-side claddinglayer side from a center position of the active layer.
In this fashion, if the maximum position of the emitted light intensity distribution in the active layer is shifted toward the p-side cladding layer side from the center position of the active layer so as to be equal to the maximum optical gain position in the maximum position of the emitted light intensity distribution, the optical confinement effect can be enhanced and the threshold current density can be reduced.
In this case, it is preferable that the maximum position of the emitted light intensity distribution coincides with a position of the first quantum well layer from the p-side cladding layer side of the multi-quantum well structure. In the MQW semiconductor laser using the nitride compound semiconductor, it is desired that, since the optical gain in the first quantum well layer from the p-side cladding layer side is made maximum, the maximum value of the emitted light intensity distribution should be positioned in the first quantum well layer. It is preferable that, in order to adjust the maximum value position of the emitted light intensity distribution, an n-side optical guiding layer is provided between the active layer and the n-type cladding layer, and a p-side optical guiding layer is provided between the active layer and the p-type cladding layer, and the forbidden bandwidth (energy band gap) of the n-side optical guiding layer is set wider than that of the p-side optical guiding layer.
In the nitride compound semiconductor, since the wider the forbidden bandwidth the smaller the refractive index, the emitted light intensity distribution is shifted towards the p-side if the forbidden bandwidth of the n-side optical guiding layer is set wider than that of the p-side optical guiding layer.
It is preferable that, in order to adjust the maximum value position of the emitted light intensity distribution, the n-side optical guiding layer is provided between the active layer and the n-type cladding layer, and in addition a thickness of the p-side optical guiding layer is set thicker than that of the n-side optical guiding layer.
Asymmetry of the light guide structure can be formed by providing the thickness of the p-side optical guiding layer and the n-side optical guiding layer in an SCH structure asymmetrically. Also, the emitted light intensity distribution can be shifted toward the p side by setting the thickness of the p-side optical guiding layer thicker than that of the n-side optical guiding layer.
In order to adjust the maximum value position of the emitted light intensity distribution, the forbidden bandwidth of the n-type cladding layer may be set wider than that of the p-type cladding layer.
The emitted light intensity distribution can be shifted toward the p side by. setting the forbidden bandwidth of the n-type cladding layer wider than that of the p-type cladding layer. In this case, the forbidden bandwidth of the n-side optical guiding layer may be set wider than that of the p-side optical guiding layer, otherwise the thickness of the optical guiding layers may be set asymmetrically.
(4) According to the present invention, in the semiconductor laser using the nitride compound semiconductor, a single quantum well (SQW) structure is employed as the active layer, and an intermediate layer (i.e., electron blocking layer or overflow stop layer) which has an energy band gap wider than the active layer is provided between the active layer and the p-side optical guiding layer, and a displacement between the maximum position of the emitted light intensity distribution and the center position of the active layer due to the intermediate layer can be compensated by the n-side layer. The bandgap of the intermediate layer is wider than that of the active layer and optical guiding layer.
If the intermediate layer which having the wide energy band gap is provided between the active layer and the p-side optical guiding layer to prevent the overflow of electrons, the maximum .position of the emitted light intensity distribution is displaced toward the n side from the center position of the active layer due to the intermediate layer. However, such displacement can be compensated by making the energy band gap of the n-side layer wider, and also the maximum position of the emitted light intensity distribution can be equal substantially to the center position of the active layer.
In this event, it is preferable that the n-side layer should be composed at least of the n-side optical guiding layer and that the forbidden bandwidth of the n-side optical guiding layer should be set wider than that of the p-side optical guiding layer.
Likewise, if the forbidden bandwidth of the n-side optical guiding layer should be set wider than that of the p-side optical guiding layer, the emitted light intensity distribution can be moved toward the p side and accordingly the maximum position of the emitted light intensity distribution can coincide substantially with the center position of the active layer.
Also, it is preferable that the n-side layer should be composed at least of the n-side optical guiding layer and that the thickness of the n-side optical guiding layer should be set thinner than that of the p-side optical guiding layer.
In this manner, if asymmetry is introduced by setting the thickness of the n-side optical guiding layer thinner than that of the p-side optical guiding layer, the emitted light intensity distribution can be shifted toward the p side and therefore the maximum position of the emitted light intensity distribution can substantially coincide with the center position of the active layer.
In this case, it is preferable that the n-side layer should be composed at least of the n-type cladding layer and that the forbidden bandwidth of the n-type cladding layer should be set wider than that of the p-type cladding layer.
In this fashion, the emitted light intensity distribution can be shifted toward the p side by setting the forbidden bandwidth of the n-type cladding layer wider than that of the p-type cladding layer and thus the maximum position of the emitted light intensity distribution can substantially coincide with the center position of the active layer. In this case, the forbidden bandwidth of the n-side optical guiding layer may be set wider than that of the p-aide optical guiding layer, otherwise the thickness of the optical guiding layers may be set asymmetrically.
Further, it is preferable that the well layer constituting the active layer in the above quantum well structure should be formed of AlxGayIn1xe2x88x92xxe2x88x92yN (where 0xe2x89xa6x less than 1, 0 less than yxe2x89xa61).
It is preferable that, as the well layer constituting the active layer in the above quantum well structure in the short wavelength semiconductor laser, the nitride compound semiconductor such as GaN, InGaN, AlGaN, AlInGaN, or the like, i.e., AlxGayIn1xe2x88x92xxe2x88x92yN (where 0xe2x89xa6x less than 1, 0 less than yxe2x89xa61) shouldbe employed.
(5) According to the present invention, in the semiconductor laser using the nitride compound semiconductor, the p-side optical guiding layer is composed of either InGaN or GaN and the impurity concentration of the p-side optical guiding layer is set to below 1xc3x971017 atoms/cm3.
In this way, in case the p-side optical guiding layer (low impurity concentration layer) made of InGaN or GaN whose impurity concentration of the p-side optical guiding layer is set to below 1xc3x971017 atoms/cm3 is employed, the hole mobility in the p-side optical guiding layer can be enhanced and also the hole injection efficiency can be improved. In addition, since the crystal quality of the optical guiding layer can be improved by reducing the impurity concentration of the optical guiding layer, non-radiative recombination can be reduced. As a result, the threshold current density Jth can be made low.
In this case, it is preferable that the p-side optical guiding layer should be composed of the undoped layer, Moreover, it is preferable that the hole mobility in the p-side optical guiding layer should be set in excess of 2 cm2/Vxc2x7s .
As described above, when the hole mobility in the low impurity concentration layer (p-side optical guiding layer) becomes higher, holes can be injected more effectively, and the hole mobility of more than 2 cm2/Vxc2x7s which is considered at present to enable the laser oscillation is needed.
(6) According to the present invention, in the semiconductor laser using the nitride compound semiconductor, the thickness of the p-side optical guiding layer is set thinner than that of the n-side optical guiding layer.
Like this, non-radiative recombination in the p-side optical guiding layer can be reduced by setting the thickness of the p-side optical guiding layer thinner than that of the n-side optical guiding layer. As a result, the semiconductor laser having the low threshold current density Jth can be constructed.
Also, according to the present invention, in the semiconductor laser using the nitride compound semiconductor, the thickness of the paide optical guiding layer is set to below 0.1 xcexcm.
In this way, if the thickness of the p-side optical guiding layer is set to below 0.1 xcexcm, more preferably less than 0.08 xcexcm (80 nm), non-radiative recombination in the p-side optical guiding layer can be reduced effectively.
It is preferable that the forbidden bandwidth (energy band gap) of the p-side optical guiding layer should be set wider than that of the n-side optical guiding layer.
Like the above, overflow of the electrons into the p-side optical guiding layer side can be prevented by setting the forbidden bandwidth of the p-side optical guiding layer wider than that of the n-side optical guiding layer.
(7) According to the present invention, in the semiconductor laser using the nitride compound semiconductor, composition of the p-side optical guiding layer is selected such that the forbidden bandwidth is made small in a region adjacent to the active layer and also the forbidden bandwidth is made large in a region adjacent to the p-type cladding layer.
If such p-side optical guiding layer is emloyed, overflow of the electron into the p-side cladding layer can be prevented while holding the sufficient optical confinement.
Either continuous (graded) change or step-like change may be adopted as the change of the forbidden bandwidth in the p-side optical guiding layer. The step-like change may be obtained by the multi-layered structure, e.g., two-layered structure and forth.
(8) According to the present invention, in the semiconductor laser using the nitride compound semiconductor, a structure in which the forbidden bandwidth (energy band gap) of the p-type optical guiding layer is made continuously smaller from the side being adjacent to the active layer toward the side being adjacent to the p-type cladding layer is employed.
In this manner, if the p-side optical guiding layer in which the forbidden bandwidth of the p-type optical guiding layer is reduced continuously smaller from the side being adjacent to the active layer toward the side being adjacent to the p-type cladding layer, i.e., the p-side inversely graded optical guiding layer, no energy spike serving as the bar for the hole injection is generated and as a result overflow of the electron can be prevented while keeping the enough hole injection.
In this case, the narrow forbidden bandwidth layer constituting the p-side optical guiding layer is formed of InGaN or GaN and the wide forbidden bandwidth layer is formed of AMGaN.
As above, it is desired that, in order to prevent overflow of the electron effectively, AlGaN should be employed as the wide forbidden bandwidth layer because xcex94Ec/xcex94Eg on a GaN/AlGaN interface or an InGaN/AlGaN interface is large.
The xcex94Ec is a energy discontinuity of the conduction band and the xcex94Eg is a difference in band gap.
(9) According to the present invention, in the semiconductor laser using the nitride compound semiconductor, the photoluminescence wavelength distribution in the active layer in the resonator is set to less than 90 meV.
Like this, in the short wavelength semiconductor laser, it is preferable that the photoluminescence wavelength distribution in the active layer in the resonator must be set to less than 90 mev to suppress the multi-wavelength oscillation, and more preferably it should be set to less than 50 meV.
It is preferable that the photoluminescence wavelength distribution in the active layer in the resonator should be set to less than 50 meV.
(10) According to the present invention, in the semiconductor light emitting device using the nitride compound semiconductor, the dislocation density in the active layer in the resonator is set to less than 1xc3x97109 cmxe2x88x922.
In the prior art, the dislocation density in the short wavelength semiconductor laser is on the order of 109 cmxe2x88x922, i.e., 1xc3x97109 cmxe2x88x922 to 1xc3x971010 cmxe2x88x922. The magnitude or level of unevenness of the PL (photoluminescence) peak wavelength coincides with the dislocation interval.
Therefore, in the present invention, the dislocation density in the active layer in the resonator is set to less than 1xc3x97109 cmxe2x88x922, preferably less than 1xc3x97108 cmxe2x88x922 and more preferably less than 1xc3x97107 cmxe2x88x922, by reducing the dislocation density in the active layer in the resonator to then reduce the PL peak wavelength distribution.
More particularly, in the short wavelength semiconductor laser in the prior art, since the sapphire whose lattice mismatching is very large such as about 13% has been employed as the growth substrate, the dislocation density in the active layer in the resonator has been about 1xc3x971010 cmxe2x88x922. Nevertheless, in the nitride compound semiconductor, it has been said that the dislocation has no influence upon the device characteristics since dislocation does not form the non-radiative center. For this reason, regardless of the dislocation density, development of the devices employing the nitride compound semiconductor have been proceeded.
However, the dislocation density and the uneven composition have a correlation. The uneven composition decreases when the dislocation density is reduced. By employing the SiC (silicon carbide) substrate, the lattice mismatching can be significantly reduced like 3%, As a result, the dislocation density in the active layer can be reduced less than 1xc3x971010 cmxe2x88x922, and reduced at least on the order of 107 cmxe2x88x922. For the above reason, the short wavelength semiconductor light emitting device in which the multi-wavelength oscillation is suppressed can be practically implemented.
It is preferable that the dislocation density in the active layer in the resonator should be set to less than 1xc3x97108 cmxe2x88x922.
Also, it is preferable that In should be contained in the active layer as a constituent element.
In this fashion, if the semiconductor in which In should be contained as a constituent element is employed as the active layer, especially if InGaN whose in composition ratio is relatively high Is employed, an active layer having excellent crystal quality, which is suited for a blue-light emitting device, especially a blue-light emitting semiconductor laser, can be implemented by satisfying the above conditions of the luminescence wavelength and the dislocation density.
(11) According to the present invention, in the semiconductor laser using the nitride compound semiconductor, the growth rate used in growing the active layer is set to more than 0.1 xcexcm/hour.
In the nitride compound semiconductor, if the growth rate used in growing the active layer is set to less than 0.1 xcexcm/hour, the PL wavelength distribution, i.e., a PL peak wavelength distribution becomes large. Under a growth rate for the active layer of 0.075 xcexcm/hour, the PL peak wavelength distribution increases, especially when In composition of InGaN increases. Consequently, the laser oscilatiqn becomes impossible Accordingly, since the PL peak wavelength distribution can be reduced less than 90 meV by employing the growth rate of more than 0.1 xcexcm/hour, the semiconductor laser in which the multi-wavelength oscillation is suppressed can be fabricated with good reproducibility and the blue-light emitting device which has a narrow half width and a high impurity as an LED can be practically implemented.
It is preferable that the growth rate should be set to more than 0.2 xcexcm/hour.
In this way, if the growth rate is increased, the PL peak wavelength distribution can be made narrower, so that suppression of the multi-wavelength oscillation can be made easy.
It is preferable that the growth rate should be set to more than 0.3 xcexcm/hour especially.
As above, in case the growth rate is set to more than 0.3 xcexcm/hour, when the InGaN having the relatively large In composition ratio which is suitable for the blue-light emitting device is to be grown, the increase of the PL peak wavelength distribution does not occur and the active layer which has the narrow PL peak wavelength distribution of less than 90 meV, for example, can be grown with good reproducibility and the active layer which has high PL light intensity and good crystal quality can be grown.
It is preferable that the SiC substrate should be employed as the substrate and a surface of the SiC substrate should be etched.
In other words, since distribution of photoluminescence wavelength (i e., dislocation density of the active layer) caused by uneven composition depends upon surface defect density of the substrate, the active layer with the small dislocation density can be grown with reproducibility by removing the surface defect by means of etching, especially dry etching of the surface even if the SiC substrate is employed.
It is preferable that In should be contained in the active layer formed over the substrate as a constituent element.
In other words, the manufacturing method of the present invention is especially effective for the semiconductor light emitting device having the active layer in which In is contained as the constituent element.
(12) According to the present invention, in the semiconductor laser using the nitride compound semiconductor, the p-type cladding layer is composed of a multi-layered structure in which an intermediate layer having a narrow forbidden bandwidth is put between two p-type semiconductor layers each having a wide rorbidden bandwidth.
By putting the intermediate layer with narrow band gap into the p-cladding layer, electrons in the p-cladding layer due to the overflow are trapped in the intermediate layer, and recombine radiatively. Since radiative recombination does not generate heat, as this emits energy as light, the positive feedback of overflow and heat generation is supressed. Consequently, the threshold current density for laser oscillation can be reduced.
It is preferable that the intermediate layer 6 is composed of a single layer having a narrow forbidden bandwidth.
Such intermediate layer may be composed of the single layer having a narrow forbidden bandwidth. In this case, it is desired that p-type impurity should be doped into the layer to reduce resistance of the p-type cladding layer.
Also, it is preferable that the forbidden bandwidth should be changed continuously in the intermediate layer such that the lowest forbidden bandwidth is given at any position between two p-type semiconductor layers each having a wide forbidden bandwidth.
In this way, if composition of material of the intermediate layer is changed to have a substantially U-shaped forbidden bandwidth distribution, hole injection into the p-side optical guiding layer can be made smooth.
It is preferable that the intermediate layer is composed of the multi-layered structure in which a plurality of wide forbidden bandwidth layers and narrow forbidden bandwidth layers are laminated alternatively.
Like the above, the intermediate layer may be composed of the multi-layered structure. In this case, crystal quality can be improved by using undoped narrow forbidden bandwidth layers, so that radiative recombination probability in the narrow forbidden bandwidth layers can be enhanced.
Also, in this case, it is preferable that the forbidden bandwidth located between the wide forbidden bandwidth layers and the narrow forbidden bandwidth layers constituting the intermediate layer should be changed continuously.
In this manner, the holes can be injected into the p-side optical guiding layer smoothly by changing the forbidden bandwidth located between the wide forbidden bandwidth layers and the narrow forbidden bandwidth layers constituting the intermediate layer continuously.
It is preferable that at least a part of the narrow forbidden bandwidth layers in the intermediate layer should be formed as an undoped layer.
In this fashion, if at least a part of the narrow forbidden bandwidth layers in the intermediate layer is formed as an undoped layer, the crystal quality can be improved and also the radiative recombination probability can be enhanced.
It is preferable that, in the p-type semiconductor layer or the intermediate layer, the wide forbidden bandwidth layers should be formed of AlGaN and the narrow forbidden bandwidth layers should be formed of InGaN, GaN, or AlGaN.
(13) According to the present invention, in the multi-quantum well structure semiconductor laser using the nitride compound semiconductor, the thickness of the barrier layer constituting the multi-quantum well active layer is set to below 5 nm.
In the nitride compound semiconductor, since a penetration of the wave function from the well layer is small, the problem of reduction of the optical gain does not become so serious even if the thickness of the barrier layer constituting the multi-quantum well active layer is made thin. Therefore, the characteristics of the multi-quantum well structure semiconductor laser can be improved if inhomogeneous carrier injection among wells is improved by setting the thickness of the barrier layer constituting the multi-quantum well active layer to below 5 nm.
In this case, it is preferable that the barrier layer constituting the multi-quantum well active layer is formed of InGaN having the In composition ratio of more than 0.04 As the composition ratio of In in InGaN is larger, the energy bandgap of the InGaN is smaller, and the strain of the barrier layer made of the InGaN is larger. The strain of the barrier layer having the composition ratio set more than 0.04 can be smaller than the critical strain value by thinning the thickness of the barrier layer. The carriers inject to the wells of the multi-quantum well structure efficiently in such the composition ratio of In over 0.04.
(14) According to the present invention, in the semiconductor laser using the nitride compound semiconductor, an impurity concentration of the electron blocking layer provided on the p side of the active layer is set to below 1.0xc3x971017.
In this case, it is preferable that the electron blocking layer provided on the p side of the active layer is formed of the undoped layer.
If the electron blocking layer provided on the p side of the active layer is formed of the layer in which the impurity concentration is below 1.0xc3x971017 atoms/cm3, especially, the undoped layer, the holes are not scattered by the impurity. Therefore, the effective hole mobility can be improved, the hole injection efficiency can be improved, and the applied voltage can be lowered.
(15) According to the present invention, in the semiconductor laser using the nitride compound semiconductor, a atructure in which the forbidden bandwidth in the p-side region of the electron blocking layer adjacent to the p side relative to the active layer is changed gradually is employed.
In this manner, the electron affinity between the electron blocking layer and the p-aide layer (i.e., the p-side optical guiding layer or. the p-side cladding layer) can be changed gradually by changing the forbidden bandwidth gradually in the p-side region of the electron blocking layer. As a result, generation of notches which act as potential barriers on an interface between the electron blocking layer and the p-side layer can be suppressed. If generation of the notches can be suppressed, the hole injection efficiency can be improved and the applied voltage can be made small. Both the continuous change and the step-like change are included in the change of the forbidden bandwidth.
Also, according to the present invention, in the semiconductor laser using the nitride compound semiconductor, the forbidden bandwidth in the n-side region of the electron blocking layer adjacent to the p side of the active layer, which is close to the active layer, is changed gradually.
In this manner, the electron affinity between the electron blocking layer and the active layer can be changed gradually by changing. the forbidden bandwidth gradually in the n-side region of the electron blocking layer. As a result, since generation of notches which act as potential barriers on an interface between the electron blocking layer and the active layer can be suppressed, the hole injection efficiency can be improved and the applied voltage V can be made small. Both the continuous change and the step-like change are included in the change of the forbidden bandwidth.
In the present invention, in the semiconductor laser using the nitride compound semiconductor, both forbidden bandwidths in the n-side region and the p-side region of the electron blocking layer adjacent to the p side of the active layer are changed gradually.
In this way, the electron affinity between the electron locking layer and the active layer and the electron affinity etween the electron blocking layer and the p-side layer can be changed gradually by changing the forbidden bandwidths gradually in the n-side region and the p-side region of the electron blocking layer respectively. As a result, since generation of notches which act as potential barriers on both interfaces can be suppressed, the hole injection efficiency can be improved and also the applied voltage V can be reduced.
(16) According to the present invention, in the semiconductor laser using the nitride compound semiconductor, a Mg concentration in the electron blocking layer provided in the p-side region of the active layer is set to more than 7xc3x971019 atoms/cm3.
Like the above, by setting the Mg concentration in the electron blocking layer provided in the p-side region of the active layer to more than 7xc3x971019 atoms/cm3, overflow of the carrier can be suppressed effectively based on evaluation of the emission spectrum.
The reason, though not made fully clear, may be supposed like that impurity level is formed at high density in the valence band side of the electron blocking layer and then, since the holes are injected into the active layer via the impurity conduction (or hopping conduction) or the tunnel conduction through the impurity-level concentration is so high that holes are injected into the active layer via the impurity levels by hopping conduction or tunneling conduction. As a result, the hole injection efficiency is improved.
(17) According to the present invention, in the method of manufacturing the semiconductor laser using the nitride compound semiconductor, a growth temperature of the electron blocking layer adjacent to the p side of the active layer is set to 600 to 900xc2x0 C.
In this case, it is preferable that the growth temperature of the electron blocking layer is set to be identical to that of the active layer.
Like this, if the growth temperature of the electron blocking layer is set to 600 to 900xc2x0 C. which is lower than around 1100xc2x0 C. of the growth temperature in the prior art and is identical to that of the active layer, luminescence intensity in the active layer can be enhanced and also light emission in the p-side optical guiding layer can be reduced.
If magnesium (Mg) is doped in the electron blocking layer, it is preferable that Mg should be doped such that the concentration exceeds 7xc3x971019 atoms/cm3.
The electron blocking layer having such Mg concentration and provided on the p side of the active layer can suppress the overflow of the carrier from the active layer effectively in an evaluation of emission spectrum.